This fixes building resource files that refer to external files
relative to the source directory, without specifying any directory
via include flags to windres itself. This is needed since llvm-rc
doesn't see the original source file but only sees the preprocessed
one in a temporary directory.
Alternatively, the preprocessed file could be placed in the same
directory as the source file, but when building e.g. outside of the
source tree, writing files (even temporary ones) inside the source
tree isn't generally expected.
This makes the output architecture independent, allowing
the x86_64 GNU binutils binary to be used for other
architectures as well, including arm/arm64 which aren't supported
by binutils.
Martin Herkt